It's not a bulb, but is it a geophyte?
ConroeJoe@aol.com (Sun, 23 Jan 2005 09:50:26 PST)

Hi,

One cactus that I grow produces underground tubers (I suppose they are tubers
and not rhizomes, etc.). The root system reminds me of a Dahlia, but the
enlarged roots tend to be long and cylindrical in shape.

LINK: Opuntia macrorhiza and roots (pads shriveled for winter)
http://members.aol.com/PlantStudy/omac/…

Cordially,

Joe Shaw